Piezometer



Oct. 26, 1937. H. J. scHRADER 2,096,826

PIEZOMETER Filed Feb. 29, 1936 18 20 ZZ 22a 53' AMR Patentedoct. 26,1.937 4 UNITED STATES PATENT OFFICE PIEZOMETER Harold J. Schrader,Haddon Heights, N. J., as-

slgnor to Radio Corporation o! America, a corporation of DelawareApplication February 29, 1936. Serial No. ,416

10 Claims. (Cl. 177-351) 'I'his invention relates to piezometers andpar- An object, therefore, of the present invention ticularly toapparatus for measuring instantais to provide an inertia-lesspiezometer. neous and/or continuously variable pressures by Anotherobject of the invention is to provide a utilization of the so-calledpiezo-electric properselfinsulating piezometer. ties characteristic ofcertain crystals, such for Another object of the invention is to providea instance as quartz, tourmaline, etc. Simple, compact, accurate,inexpensive and trou- Piezometers or "piezo-electric gauges as theyble-free piezometer. are sometimes called nd useful application in theOther objects and advantages will be apparent automotive, ordnance andsmall arms industries, and the invention itself will be best understood1J among others, where they are employed in conby reference to thefollowing specication and to 10 junction with suitable indicatingdevices (such the accompanying drawing wherein for instance as a cathoderay oscillograph) to Fig. 1 is a cross-sectional view of a piezometerobserve time-pressure phenomena. Usually the constructed in accordancewith lthe invention, instrument itself comprises a cylindrical casingFig. 2 is an electrical diagram showing the incontaining two or moreparallelly-connected strument of Fig. 1 in circuit with an amplier andl.; quartz crystals and a pendant plunger one end oi.' cathode-rayoscillograph, and which is maintained against the crystal pile, as Fig.3 is a view partly in section showing the by a spring, while the bodyand opposite end of instrument of Fig. 1 applied to a cylinder of an theplunger are inserted in a bore of suitable internal combustion engine. ogauge formed for that purpose in the wall of the In the drawing IIJdesignates generally a 20 pressure-chamber to be examined. The cylinderthreaded, crystal-containing, casing which is containing the crystals isprovided with a screw adapted to be screwed into a bore in the wall ofarrangement for adjusting the pressure on the a pressure-chamber I2(Fig. 3). A number of crystals and with suitable clamping means forlock-nuts I4 are preferably provided to maintain aiixing theunit to theexterior of the barrel of a the casing I0 in position with its leadingend ush 25 gun or the block of an engine. Preferably the with the Wallof the chamber. The end of casing plunger is of a length such that itsleading end I0, presented to the interior of this chamber I2, is ushwith the inner Wall of the pressure chamis Closed by a diaphragm I6which may cofnber so that it presents no obstruction to the Veniently beconstituted by a disc of stainless "U missile'and in no-wise alters thevolumetric area steel. say 115 thick. Freely mounted within the 30 ofthe pressure chamber. casing I0 is a metal hemisphere I 8 with itscurved An explosion such as one emitted by the firing surface contiguousdiaphragm I6, and its plane of a shell or'the ignition of a vapor chargeexerts Surface supporting a piezo-electric crystal 20. A a force againstthe chamber end of the plunger second crystal 22, is separated fromcrystal 2li 3:, which is transmitted by the plunger through the by ametal electrode 2|. Both crystals 2li and 22 35 wall of the chamber tothe external crystals which are so cut with respect to their naturalcrystalloin turn translate this mechanical rforce into elecgraphic axesthat when subject to tension or trical energy' in the manner peculiar topiezopressure they will develop electric charges of simielectricsubstances. The electrical energy thus lar polarity adjacent theirrespective opposite 4u developed is applied through a suitable amplierelectrode surfaces. Thus, as indicated in Fig. 2, 40 to a cathode raytube or the like to produce a that electrode face, of each crystal 20,22 which visual indication for the forces at work within is contiguousthe electrodes 2l are of the saineI the explosion chamber. polaritywhile the charge adjacent the sur- Many valuable advances have been madein faces remote from electrode 2| are each of the 1i-various arts as aresult of such readings. Howopposite (-1-) polarity. The crystals may ofi5 ever, careful analysis of the operation of plungercourse be so cutthat the polarities are reversed, type piezometers indicates that thereadings are i. e. With the positive polar faces of each crystal lessaccurate than is desirable because the plunger toward the electrode 2 I.(which is necessarily of an appreciable mass) Each of the crystals iscut, in accordance with 50 introduces undesirable inertia factors.Further, the invention, in the form of a frustrum of a 50 if, as isusually the case, several conductors are coneand of a size suchl thatonly the perimeter employed to connect the piezo crystals in parallel ofthe larger base of each cone contacts the inner array the necessaryinsulation necessitates a more wall IM of the conductive'casing I0. Thesmallbulky and more complex assembly than is deer or "inner base of thecrystals 20, 22, are out 55 sirable. of contact with casing I0 so thatshort-circuiting 55 axis of casing l0.

- instrument.

of the tapered 'orifice :22a andpassesthrough crystal 22 Withoutcontacting that portion of its i 26 ,arer provided for maintaining thecrystal elements 20, 22, under moderate tensionagainst ythe f hemispherei8, which in turn, is pressed against thediaphragm 16. Theadjustmentneed not be y f critical, itk is made during lassembly bymeansof:

awrenchhaving prongs adapted to engage the sockets: 2st inv nut y2s.,B01-.h thebaeking vplate 24 and nut 2.6 are provided; withraligned'orices,

designated 24a, 26a, respectively, `which forma continuation of thecrystal orifice 22a and through soldered.

whichthe lead rod 23,enters the central hollow by rrneansofy a threadednut 31).y Thespaoe 10c about the washer 28 is further sealed by a..filling of glyptol or other high-temperature insulating cement.

The casing or shell IU, the diaphragm I6, the hemisphere I8 and thecentral electrode 2| are preferably formed of stainless steel. Since thetemperature coefiicient of expansion of the quartz elements 20, 22 isless than that of steel, the assembly might tend to loosen when subjectto comparatively high temperatures unless some compensating means isprovided. Accordingly, the backing plate 24 is formed of brass. or thelike to compensate for such unequal expansion. This plate 24 is shapedwith its side walls sloping inwardly out of full length contact with theinner wall Illa of the casing or shell I to obviate sticking and tofacilitate assembly.

With the device l0 of Fig. 1 positioned, for instance, in the wall of acylinder I2 of an internal combustion engine (Fig. 3) the pressure forceof an explosion initiated as by a spark-plug S will be exerted againstthe diaphragm I6, hemisphere I8 and the crystal assembly 20, 2|, 22. As`shown in the electrical diagram of Fig. 2 and as previously set forththe crystals are assembled with their faces of similar polarity incontact with electrode 2|, hence the charges of like sign will be added.-The sum of the combined negative charges are transferred from electrode2l through lead rod 23 (Fig. 1) and shielded cable 23a (Figs. 2 and 3)to a suitable amplifier (Fig.

2). The circuit is completed through a common connection 23h from theopposite positively polarized faces of the crystals 22, 23 which are incontact with the shell I0. The amplifier potentials are preferablyimpressed upon the vertical deiiecting plates-53, 55 of a cathode-rayoscillograph l. The tube illustrated is provided with a pair ofhorizontal deflecting plates 51-59 which This .arrangement preventsshort-cirf cuitingof: the faces vof opposite polarity by `the Theupperend of thecylinder lll is preferably .hermetlcally kvsealed as by meansof a seal constituted by an insulating washer 28oflarninated mica havinga central metal eyelet 28afinlwhich leadwire 23 is This seal isconveniently heldin place:

of the faces of opposite polarity'is avoided. f ,'I'husr casing `l0serves as one (in this casefthe posil tive) rf the two necessaryexternal leads of the v The second (in this case the .fneg'af `tlve)lead is constituted by a preferablyrigid wireor rod 23 which yextendsthrough a taperedv oriiicenaiincrystal 22and=thence along the long Asshown inFig. 1 rod 2 3 cion nects with'electrode 2l adjacent the smallend yare. energized, for the purpose of providing a time-axis, by asaw-tooth r oscillator: 6l yand 1ampliflerv63. Preferably,thefrequencyof oscilla minimum inertia,ythis feature togetherwith the ff f other features of novelty of the self contained,

self-insulated, hermeticallyl sealedy plezometer ofv f the inventionlensure an accurate `indication of the time-,pressure phenomena takingplace inthe pressure chamber itcy which kit is applied.: `As otherrapplications and embodiments .of the invention will suggest themselvesto those skilled in the art,it1is rtobeunderstoodthat, the scope 1 'ofinvention; is not to be limited except asire-y quired by the yprior artand kby the' spirit vof the appendedclaims. y f f I claim: f 1. vIncombinationy a electrode faces and a vconductive leadfrom said electrodeextending through lsaid, o'ricey out of contact with the other of saidpolarized faces.

v2. The invention as set forth in claim 1 char quartz piezcelectric elefment having oppiositely polarized electrode faces lying inv parallelplanes, an orifice iny said quartz element extending in a directionnormal to theL planesjin whichr .saidl polarized electrode faces f,vlie, anelectrode adjacent one of said polarized 'M il acterized in thatsaidr orifice is taperedin: cross section and said rconductorconnectswith said electrode adjacent the small end ofsald orifice.

3. In aidev'ice ofthe Iclass described, ahollow electrically conductivemember, ra piezo-electric velement* therein f having oppositelypolarized elec trode faces: they innerl wall,r of said conductiveymember contacting said element adjacent only one rof its said polarizedfaces.

4. The invention as set forth in claim 3 characterized in that saidpiezo-electric element is in the form of a frustrum of a cone.

5. In a device of the class described, a hollow electrically conductivemember, a pair of piezoelectric elements therein each having oppositelypolarized electrode faces, an electrode interposed between similarlypolarized of said electrode faces, the perimeters of certain of saidpolarized faces contacting the inner wall of said conductive member andthe perimeters of the other of said polarized faces and of saidelectrode being out of contact with said wall.

6. In a device of the class described, a hollow electrically conductivemember, a pair of piezoelectric elements therein each having oppositelypolarized electrode faces, an orifice in one of said elements, anelectrode interposed between similarly polarized of said electrodefaces, a conductor connected to said electrode and extending throughsaid orifice out of contact with the other polarized face of said oricedelement.

7. The invention as set forth in claim 6 characterized in that saidother polarized face loi? said oriiiced piezo-electric element and thecorresponding face of the other piezo-electric element.

connect with said hollow member ,whereby said 'member constitutes areturn lead for electric charges appearing upon the faces of saidelements which are adjacent said electrode.

8. In a device of the class described, a hollow element contacting theinner wall of said hollow member, an electrode interposed between the ysmall ends of said frusta-conical elements out of contact with saidinner wall, an orice in one of said elements, a conductor connected tosaid electrode and extending through said oriiice to the exterior ofsaid hollow member out of contact with the base of said oriced elementand the inner wall of said hollow member.

9. A piezometer comprising an electrically conductive cylinder, adiaphragm closing the leading end of said cylinder, a pressure transfermember contacting the inner wall of said cylinder and said diaphragm, apair of frusto-conical piezoelectric elements and an interposedelectrode, the base of one of said elements contacting said pressuretransfer member and the perimeter of the base of each element contactingthe inner Wall of said cylinder, the small ends of said elements andsaid electrode being out of contact Y with said wall, a second electrodeadjacent the base of the element which is remote from said diaphragm,and a conductor extending from said first mentioned electrode throughsaid last mentioned piezo-electric element and through said secondmentioned electrode to the exterior of said cylinder and out of contactwith said element, second electrode and cylinder.

10. The inventicn as set forth in claim 9 Wherein said cylinder, saidpressure transfer member and said iirst mentioned electrode are composedof a material having a certain coeicient `of expansion and saidpiezo-electric elements and said second electrode are composed ofmaterials having a combined coeicient of expansicn which is l

